1. Field of the Invention
The invention relates to an apparatus for reducing a magnetic unidirectional flux component in the core of a transformer having at least three limbs, in particular a three-phase transformer, comprising at least one compensation winding per limb of the transformer, where the compensation windings are magnetically coupled to the core of the transformer.
The area of application of the invention in principle relates to transformers in the low or medium voltage range, as well as very high power transformers (power transformers, HVDCT (high voltage DC transmission) transformers).
2. Description of the Related Art
In the case of electrical transformers, as used in power distribution grids, a direct current may undesirably be fed into the primary winding or secondary winding. This type of direct current feed, also called a DC component, may be caused, for example, by electronic components, as used nowadays to activate electrical drives or in reactive power compensation. Another cause could be “geomagnetically induced currents” (GIC).
Because of solar winds, the Earth's magnetic field fluctuates, meaning that very low-frequency voltages are induced in conductor loops at the Earth's surface. In the case of long electrical power transmission lines, the induced voltage can bring about relatively large low-frequency currents (quasi-direct currents). Geomagnetically induced currents occur approximately in ten-year cycles. They are distributed evenly across all (three) phases, can reach up to 30 A per phase and discharge via the star point of a transformer. This results in considerable saturation of the core of the transformer in a half-cycle and, hence, in a strong excitation current in a half-cycle. This additional excitation has a strong harmonic component and as a result eddy current losses are caused in windings and iron parts of the transformer by the stray field with a harmonic component. This can lead to local overheating in the transformer. Furthermore, because of the large excitation requirement there is a high reactive power consumption and voltage drop. Together, this can lead to instability of the power transmission grid. In very simplified terms, the transformer behaves in a half-wave like a choke.
Hence, in the specification of transformers many power transmission companies already require 100 A direct current for the star point of the transformer.
According to WO 2012/041368 A1, an electrical voltage induced in a compensation winding is used and is utilized to compensate for the disruptive magnetic unidirectional flux component, by connecting a thyristor switch in series with a current-limiting inductor, in order to introduce the compensation current into the compensation winding. This solution works well for direct currents to be equalized in a range that is an order of magnitude smaller than geomagnetically induced currents, in other words approximately in the range below 10 A. For geomagnetically induced currents, it would be necessary to go to the medium voltage level, i.e., to the range of approximately 5 or 8 kV, and to deploy high-capacity thyristors. Because of the high power loss of such thyristors separate, cooling for the thyristors would have to be provided, so that this solution would then not be economic.
Another solution for geomagnetically induced currents is the “DC blocker” in which, in principle, a capacitor is connected into the star point of the transformer. This solution is problematic, because charging the capacitor gives rise to a displacement voltage. In addition, the displacement voltage is limited at the capacitor, so that generally it is not possible to block the entire direct current. This solution is also problematic in the event of a short-circuit in the transmission grid, and hence zero currents.